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There's a species of sea slug, Elysia chlorotica, that
only needs to eat when it is very young.

It gorges on algae, stealing their ability to
harness the sun's energy, and then proceeds to sunbathe for
the rest of its 10-month life.

But it's hardly alone: Other animals also take on some traits of
plants in order to give them an edge against competitors. From
solar-powered aphids to algae-embedded salamanders, these animals
serve as living biology lessons and could be used to better
understand immunity and improve gene therapy.

Here's a look at the weird ways some animals partner with and
mimic plants.

Symbiosis

If you can't do the things a plant can, then it's best to make
nice and establish a symbiotic relationship with photosynthetic
microorganisms. This is the trick used by many species of coral:
They provide the skeleton and housing, and the sun-powered
microbes called zooxanthellae provide the energy. [ Extreme
Life on Earth: 8 Bizarre Creatures ]

But coral aren't the only organisms that make friends with algae.
There is one symbiotic relationship that is strangely intimate —
and even more perplexing. Every spring in the northeastern United
States, spotted salamanders awake from their subterranean slumber
and gather in pools to breed. "They basically have orgies where
they lay their clutches of eggs," said Ryan Kerney, who studies
amphibians at Gettysburg College. "A couple days later, all the
eggs will turn a slight green tinge."

What's this all about, he wondered? It turns out that a certain
type of algae lives in these eggs, and confers a slight benefit
to the developing embryo by increasing the concentration of
oxygen in their cells. Strangely, though, Kenney found that these
algae actually
burrow inside the cells of the embryo. It's the first time a
symbiotic organism has been shown to penetrate the cells of a
vertebrate, he said.

"This isn't supposed to happen," he added. That's because
vertebrates have an immune system that usually attacks foreign
organisms.

Solar-powered sea slugs

Then there are sacoglossan sea slugs, several species of which
can
steal the chloroplasts of algae and photosynthesize
themselves. This is very strange indeed, since chloroplasts
need constant upkeep by the molecular machinery within algae and
plants. Somehow, the slug has found out how to keep them running
in an alien body.

"This just should not work on the face of it, but it does," said
Sidney Pierce, a biologist at the University of South Florida.
Pierce has spent much of the last four years looking for genes
that could explain how these chloroplasts function. Within the
cells of Elysia chlorotica, he's found about 50 genes
involved in photosynthesis.

How are genes transferred from algae to the slug? "If I knew
that, I'd have figured out how gene therapy works and I'd be a
millionaire and retired," Pierce said. Gene therapy involves
inserting genes into human DNA, and has the potential to help
treat everything from cancer to blindness. It has proven elusive,
however, due in part to the difficulty of inserting foreign DNA
into the human genome and getting it to function as desired.

Others are not so sure that Pierce has proven how these
sea slugs work their magic. Researchers Mary Rumpho, at the
University of Connecticut, and Heike Wägele, at Germany's Centre
for Molecular Biodiversity Research, both question his findings.
They said they aren't convinced the genes he's found have
inserted themselves into the slug's DNA. Furthermore, many more
than 50 genes would be necessary to keep these chloroplasts
running, they said.

Wägele said she thinks the solution has to do with the slug's
behavior, rather than its genes. These slugs shield the
chloroplasts with curtainlike flaps on its body called parapodia
to make them last longer, she said. The chloroplasts themselves
are also unique, and much more long-lasting than most, she added.
[ Real
or Fake? 8 Bizarre Hybrid Animals ]

"The current state of knowledge is that we don't know how they do
it," Rumpho said.

A study published earlier this year in the journal Scientific
Reports found that when placed in the light, pea
aphids can produce adenosine triphosphate, or ATP, the
cellular energy currency that powers biochemical reactions. (For
animals, cells typically convert food into ATP, while plants
create ATP via photosynthesis.)

Pea aphids are already special because they make carotenoids,
which are usually produced by plants and microorganisms, and
which can act as antioxidants when consumed by humans. These
carotenoids help determine the color of the aphids and are also
capable of making ATP from sunlight, wrote study author Alain
Robichon, of France's Sophia Agrobiotech Institute, in an email.

Growing leaves

It's easy to marvel at the wizardry of these solar-powered
animals, and natural to wonder: What can humans learn from their
tricks? Could we ever employ these techniques? Scientists said
that studies of "photosynthetic" animals could help improve
gene therapy ; if humans figure out how algal genes were
transferred into the slugs, it could perhaps help us transfer
useful genes from other creatures into our own DNA, Pierce said.

Rumpho said this research could help understand how slugs and
salamander embryos know not to attack these foreign organisms,
which could shed light on the workings of the immune system.

Unfortunately, it may however be awhile before we can make
ourselves photosynthetic and sunbathe instead of eating; animals
need much more energy than plants to move about — and making a
human's skin photosynthetic, for example, would provide a
negligible energy boost, Pierce said.

That is, unless we drastically altered our bodies to resemble
trees. "We'd all have to grow leaves if we wanted to do it," he
said.